Method for producing water-absorbing polymer particles by polymerizing droplets of a monomer solution

ABSTRACT

A process for producing water-absorbing polymer particles by polymerizing droplets of a monomer solution in a surrounding gas phase in a reaction chamber, wherein the monomer solution is metered into the reaction chamber via at least one bore, and the diameter is from 210 to 290 μm per bore and the metering rate is from 0.9 to 5 kg/h per bore.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase of International Application No.PCT/EP2009/058246, filed Jul. 1, 2009, which claims the benefit ofEuropean patent Application No. 08159844.3, filed Jul. 7, 2008.

The present invention relates to a process for producing water-absorbingpolymer particles by polymerizing droplets of a monomer solution in asurrounding gas phase in a reaction chamber, wherein the monomersolution is metered into the reaction chamber via at least one bore.

The production of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

Being products which absorb aqueous solutions, water-absorbing polymersare used to produce diapers, tampons, sanitary napkins and other hygienearticles, but also as water-retaining agents in market gardening.Water-absorbing polymers are also referred to as “superabsorbentpolymers” or “superabsorbers”.

Spray polymerization allows the process steps of polymerization anddrying to be combined. In addition, the particle size can be set withincertain limits by suitable process control.

The production of water-absorbing polymer particles by polymerizingdroplets of a monomer solution is described, for example, in EP 0 348180 A1, EP 0 816 383 A1, WO 96/40427 A1, U.S. Pat. No. 4,020,256, US2002/0193546, DE 35 19 013 A1, WO 2008/040715 A2 and WO 2008/052971 A1.

WO 2008/040715 A2 describes a spray polymerization process with adefined residence time of the initiator in the monomer solution beforethe generation of droplets.

WO 2008/052971 A1 describes a spray polymerization process with specifictemperature regulations.

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles by polymerizing dropletsof a monomer solution in a gas phase surrounding the droplets.

More particularly, it was an object of the present invention to providea process for producing water-absorbing polymer particles with narrowmonomodal particle size distribution and high density.

The object is achieved by a process for producing water-absorbingpolymer particles by polymerizing droplets of a monomer solutioncomprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) at least one crosslinker,-   c) at least one initiator and-   d) water,    in a surrounding gas phase in a reaction chamber, the monomer    solution being metered into the reaction chamber via at least one    bore, wherein the diameter is from 210 to 290 μm per bore and the    metering rate is from 0.9 to 5 kg/h per bore.

The metering rate per bore (in kg/h) is preferably at least−1.9269·10⁻⁷ x ³+2.3433·10⁻⁴ x ²−5.4364·10⁻² x+3.7719where x is the diameter per bore (in μm).

The metering rate per bore (in kg/h) is preferably at most5.5158·10⁻⁸ x ⁴−5.5844·10⁻⁵ x ³+2.0635·10⁻² x ²−3.2606x+1.8698·10²where x is likewise the diameter per bore (in μm).

The diameter of the bore and the metering rate per bore are selectedsuch that the resulting droplets have a mean diameter preferably of from300 to 700 μm, more preferably from 350 to 650 μm, most preferably from400 to 600 μm. The mean droplet diameter is determined by laserdiffraction, for example with the Malvern Insitec® S (MalvernInstruments Ltd.; Malvern; UK). With constant diameter of the bore, themean droplet diameter falls with rising metering rate per bore.

The monomer solution has, at 20° C., a dynamic viscosity of preferablyfrom 0.002 to 0.02 Pa·s, more preferably from 0.004 to 0.015 Pa·s, mostpreferably from 0.005 to 0.01 Pa·s. The mean droplet diameter rises withrising dynamic viscosity.

The monomer solution has, at 20° C., a density of preferably from 1 to1.3 g/cm³, more preferably from 1.05 to 1.25 g/cm³, more preferably from1.1 to 1.2 g/cm³.

The monomer solution has, at 20° C., a surface tension of from 0.02 to0.06 N/m, more preferably from 0.03 to 0.05 N/m, more preferably from0.035 to 0.045 N/m. The mean droplet diameter increases with risingsurface tension.

The temperature of the monomer solution as it passes through the bore ispreferably from 10 to 60° C., more preferably from 15 to 50° C., mostpreferably from 20 to 40° C.

The present invention is based on the finding that the mean droplet sizecan be kept constant with rising diameter of the bores andgreater-than-proportional increase in the metering rate per bore, andthe density of the resulting water-absorbing polymer particles risessimultaneously. Water-absorbing polymer particles with higher densitytake up less volume and can be metered more rapidly.

Too high a metering rate leads to a bimodal particle size distributionand hence to an elevated proportion of water-absorbing polymer particleswith undesirably small particle diameter.

The water-absorbing polymer particles are typically water-insoluble.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitablemonomer a) is, for example, acrylic acid purified according to WO2004/035514 A1 comprising 99.8460% by weight of acrylic acid, 0.0950% byweight of acetic acid, 0.0332% by weight of water, 0.0203% by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The acid groups of the monomers a) are typically partly neutralized,preferably to an extent of from 25 to 85 mol %, preferentially to anextent of 50 to 80 mol %, more preferably to an extent of 60 to 75 mol%, for which the customary neutralizing agents can be used, preferablyalkali metal hydroxides, alkali metal oxides, alkali metal carbonates oralkali metal hydrogencarbonates, and mixtures thereof. Instead of alkalimetal salts, it is also possible to use ammonium salts. Sodium andpotassium are particularly preferred as alkali metals, but veryparticular preference is given to sodium hydroxide, sodium carbonate orsodium hydrogencarbonate and mixtures thereof. Typically, theneutralization is achieved by mixing in the neutralizing agent as anaqueous solution, as a melt, or preferably also as a solid. For example,sodium hydroxide with a water content significantly below 50% by weightmay be present as a waxy material with a melting point above 23° C. Inthis case, metered addition as piece material or a melt at elevatedtemperature is possible.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone monoethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, preferably at least 10 ppm by weight, more preferably at least30 ppm by weight, especially around 50 ppm by weight, of hydroquinonemonoether, based in each case on the unneutralized monomer a). Forexample, the monomer solution can be prepared by using an ethylenicallyunsaturated monomer bearing acid groups with an appropriate content ofhydroquinone monoether. The hydroquinone monoethers may, however, alsobe removed from the monomer solution by absorption, for example onactivated carbon.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450A1, mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraalloxyethane, methylenebismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol, especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably from 0.01 to 1.5% by weight,more preferably from 0.05 to 1% by weight, most preferably from 0.1 to0.6% by weight, based in each case on monomer a). With risingcrosslinker content, the centrifuge retention capacity (CRC) falls andthe absorption under a pressure of 21.0 g/cm² (AUL0.3psi) passes througha maximum.

The initiators c) used may be all compounds which disintegrate into freeradicals under the polymerization conditions, for example peroxides,hydroperoxides, hydrogen peroxide, persulfates, azo compounds and whatare known as redox initiators. Preference is given to the use ofwater-soluble initiators. In some cases, it is advantageous to usemixtures of various initiators, for example mixtures of hydrogenperoxide and sodium or potassium peroxodisulfate. Mixtures of hydrogenperoxide and sodium peroxodisulfate can be used in any proportion.

Particularly preferred initiators c) are azo initiators such as2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride and2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, andphotoinitiators such as 2-hydroxy-2-methylpropiophenone and1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, redoxinitiators such as sodium persulfate/hydroxymethylsulfinic acid,ammonium peroxodisulfate/hydroxymethylsulfinic acid, hydrogenperoxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic acid,ammonium peroxodisulfate/ascorbic acid and hydrogen peroxide/ascorbicacid, photoinitiators such as1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, andmixtures thereof.

The initiators are used in customary amounts, for example in amounts offrom 0.001 to 5% by weight, preferably from 0.01 to 2% by weight, basedon the monomers a).

The water content of the monomer solution is preferably less than 65% byweight, preferentially less than 62% by weight, more preferably lessthan 60% by weight, most preferably less than 58% by weight.

The monomer solution is metered into the reaction chamber by means of atleast one bore to form droplets. The bores may be present, for example,in a dropletizer plate.

A dropletizer plate is a plate having at least one bore, the liquidentering the bore from the top. The dropletizer plate or the liquid canbe oscillated, which generates a chain of ideally monodisperse dropletsat each bore on the underside of the dropletizer plate. In a preferredembodiment, the dropletizer plate is not agitated.

The number and size of the bores are selected according to the desiredcapacity and droplet size. The droplet diameter is typically 1.9 timesthe diameter of the bore. What is important here is that the liquid tobe dropletized does not pass through the bore too rapidly and thepressure drop over the bore is not too great. Otherwise, the liquid isnot dropletized, but rather the liquid jet is broken up (sprayed) owingto the high kinetic energy. The Reynolds number based on the throughputper bore and the bore diameter is preferably less than 2000,preferentially less than 1600, more preferably less than 1400 and mostpreferably less than 1200.

The dropletizer plate has typically at least one bore, preferably atleast 10, more preferably at least 50 and typically up to 10 000 bores,preferably up to 5000, more preferably up to 1000 bores, the borestypically being distributed uniformly over the dropletizer plate,preferably in so-called triangular pitch, i.e. three bores in each caseform the corners of an equilateral triangle.

The separation of the bores is preferably from 1 to 50 mm, morepreferably from 2.5 to 20 mm, most preferably from 5 to 10 mm.

The polymerization reactor is flowed through by a gas. The carrier gascan be conducted through the reaction chamber in cocurrent or incountercurrent to the free-falling droplets of the monomer solution,preferably in cocurrent, i.e. from the top downward. After one pass, thecarrier gas is preferably recycled at least partly, preferably to anextent of at least 50%, more preferably to an extent of at least 75%,into the reaction chamber as cycle gas. Typically, a portion of thecarrier gas is discharged after each pass, preferably up to 10%, morepreferably up to 3% and most preferably up to 1%.

The oxygen content of the carrier gas is preferably from 0.5 to 15% byvolume, more preferably from 1 to 10% by volume, most preferably from 2to 7% by volume.

As well as oxygen, the carrier gas preferably comprises nitrogen. Thenitrogen content of the carrier gas is preferably at least 80% byvolume, more preferably at least 90% by volume, most preferably at least95% by volume.

The gas velocity is preferably adjusted such that the flow in thepolymerization reactor is directed, for example no convection currentsopposed to the general flow direction are present, and is, for example,from 0.01 to 5 m/s, preferably from 0.02 to 4 m/s, more preferably from0.05 to 3 m/s, most preferably from 0.1 to 2 m/s.

The gas flowing through the reactor is appropriately preheated to thereaction temperature upstream of the reactor.

The gas entrance temperature, i.e. the temperature with which the gasenters the reaction chamber, is preferably from 160 to 250° C., morepreferably from 180 to 230° C., most preferably from 190 to 220° C.

Advantageously, the gas entrance temperature is controlled in such a waythat the gas exit temperature, i.e. the temperature with which the gasleaves the reaction chamber, is from 100 to 180° C., more preferablyfrom 110 to 160° C., most preferably from 120 to 140° C.

The reaction can be carried out under elevated pressure or under reducedpressure; preference is given to a reduced pressure of up to 100 mbarrelative to ambient pressure.

The reaction offgas, i.e. the gas leaving the reaction chamber, may, forexample, be cooled in a heat exchanger. This condenses water andunconverted monomer a). The reaction offgas can then be reheated atleast partly and recycled into the reactor as cycle gas. A portion ofthe reaction offgas can be discharged and replaced by fresh gas, inwhich case water and unconverted monomers a) present in the reactionoffgas can be removed and recycled.

Particular preference is given to a thermally integrated system, i.e. aportion of the waste heat in the cooling of the offgas is used to heatthe cycle gas.

The reactors can be trace-heated. In this case, the trace heating isadjusted such that the wall temperature is at least 5° C. above theinternal reactor temperature and condensation on the reactor walls isreliably prevented.

The polymer particles can be postcrosslinked for further improvement ofthe properties. Suitable postcrosslinkers are compounds which comprisegroups which can form covalent bonds with at least two carboxylategroups of the polymer particles. Suitable compounds are, for example,polyfunctional amines, polyfunctional amidoamines, polyfunctionalepoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937736 A2, di- or polyfunctional alcohols as described in DE 33 14 019 A1,DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, asdescribed in DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

In addition, DE 40 20 780 C1 describes cyclic carbonates, DE 198 07 502A1 describes 2-oxazolidone and its derivatives such as2-hydroxyethyl-2-oxazolidone, DE 198 07 992 C1 describes bis- andpoly-2-oxazolidinones. DE 198 54 573 A1 describes2-oxotetrahydro-1,3-oxazine and its derivatives, DE 198 54 574A1describes N-acyl-2-oxazolidones, DE 102 04 937 A1 describes cyclicureas. DE 103 34 584 A1 describes bicyclic amide acetals, EP 1 199 327A2 describes oxetanes and cyclic ureas, and WO 2003/031482 A1 describesmorpholine-2,3-dione and its derivatives, as suitable postcrosslinkers.

Preferred postcrosslinkers are ethylene carbonate, ethylene glycoldiglycidyl ether, reaction products of polyamides with epichlorohydrinand mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred postcrosslinkers are2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.

In addition, it is also possible to use postcrosslinkers which compriseadditional polymerizable ethylenically unsaturated groups, as describedin DE 37 13 601 A1.

The amount of postcrosslinker is preferably from 0.001 to 2% by weight,more preferably from 0.02 to 1% by weight, most preferably from 0.05 to0.2% by weight, based in each case on the polymer particles.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the postcrosslinkersbefore, during or after the postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium.Possible counterions are chloride, bromide, sulfate, hydrogensulfate,carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate and lactate.Aluminum sulfate and aluminum lactate are preferred. Apart from metalsalts, it is also possible to use polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, from 0.001 to 1.5%by weight, preferably from 0.005 to 1% by weight, more preferably from0.02 to 0.8% by weight, based in each case on the polymer particles.

The postcrosslinking is typically performed in such a way that asolution of the postcrosslinker is sprayed onto the dried polymerparticles. After the spraying, the polymer particles coated withpostcrosslinker are dried thermally, and the postcrosslinking reactioncan take place either before or during the drying.

The spray application of a solution of the postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers and paddle mixers. However, it is also possible to spray on thepostcrosslinker solution in a fluidized bed. Particular preference isgiven to horizontal mixers such as paddle mixers, very particularpreference to vertical mixers. The distinction between horizontal mixersand vertical mixers is made by the position of the mixing shaft, i.e.horizontal mixers have a horizontally mounted mixing shaft and verticalmixers a vertically mounted mixing shaft. Suitable mixers are, forexample, horizontal Pflugschar® plowshare mixers (Gebr. LödigeMaschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixers(Hosokawa Micron By; Doetinchem; the Netherlands), Processall Mixmillmixers (Processall Incorporated; Cincinnati; US) and Schugi Flexomix®(Hosokawa Micron BV; Doetinchem; the Netherlands).

The postcrosslinkers are typically used in the form of an aqueoussolution. The content of nonaqueous solvent or total amount of solventcan be used to establish the penetration depth of the postcrosslinkerinto the polymer particles.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting behavior and reduces thetendency to form lumps. However, preference is given to using solventmixtures, for example isopropanol/water, 1,3-propanediol/water andpropylene glycol/water, where the mixing mass ratio is preferably from20:80 to 40:60.

The thermal drying is preferably carried out in contact dryers, morepreferably paddle dryers, most preferably disk dryers. Suitable dryersare, for example, Hosokawa Bepex® horizontal paddle driers (HosokawaMicron GmbH; Leingarten; Germany), Hosokawa Bepex® disk driers (HosokawaMicron GmbH; Leingarten; Germany) and Nara paddle driers (NARA MachineryEurope; Frechen; Germany). Moreover, it is also possible to usefluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream drier, for examplea shelf drier, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed drier.

Preferred drying temperatures are in the range from 100 to 250° C.,preferably from 120 to 220° C., more preferably from 130 to 210° C.,most preferably from 150 to 200° C. The preferred residence time at thistemperature in the reaction mixer or drier is preferably at least 10minutes, more preferably at least 20 minutes, most preferably at least30 minutes, and typically at most 60 minutes.

To further improve the properties, the postcrosslinked polymer particlescan be coated or subsequently moistened. Suitable coatings for improvingthe swell rate and the permeability (SFC) are, for example, inorganicinert substances, such as water-insoluble metal salts, organic polymers,cationic polymers and di- or polyvalent metal cations. Suitable coatingsfor dust binding are, for example, polyols. Suitable coatings forcounteracting the undesired caking tendency of the polymer particlesare, for example, fumed silica, such as Aerosil® 200, and surfactants,such as Span® 20. Suitable coatings for improving the color stability(yellowing stability) are, for example, reducing agents such as sodiumhypophosphite, sodium sulfite, sodium hydrogen sulfite, Brüggolite® FF6and Brüoggolite® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

The present invention further provides water-absorbing polymer particlesobtainable by the process according to the invention.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a moisture content of typically at least10% by weight, preferably at least 12% by weight, more preferably atleast 14% by weight, most preferably at least 15% by weight, andtypically less than 20% by weight.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a density of typically at least 0.55g/cm³, preferably at least 0.57 g/cm³, more preferably at least 0.59g/cm³, most preferably at least 0.6 g/cm³, and typically less than 0.75g/cm³.

The mean diameter of the water-absorbing polymer particles obtainable bythe process according to the invention is preferably from 300 to 450 μm,more preferably from 320 to 420 μm, very particularly from 340 to 400μm.

The water-absorbing polymer particles obtainable by the processaccording to the invention typically have the shape of partiallyindented hollow spheres (FIG. 1) and are approximately round, i.e. thepolymer particles have a mean sphericity (mSPHT) of typically at least0.84, preferably at least 0.86, more preferably at least 0.88, mostpreferably at least 0.9. The sphericity (SPHT) is defined as

${{SPHT} = \frac{4\pi\; A}{U^{2}}},$where A is the cross-sectional area and U is the cross-sectionalcircumference of the polymer particles. The mean sphericity (mSPHT) isthe volume-average sphericity.

The mean sphericity (mSPHT) can be determined, for example, with theCamsizer® image analysis system (Retsch Technology GmbH; Germany).

Polymer particles of relatively low mean sphericity (mSPHT) are obtainedby inverse suspension polymerization when the polymer particles areagglomerated during or after the polymerization.

The water-absorbing polymer particles produced by customary solutionpolymerization (gel polymerization) are ground and classified afterdrying to obtain irregular polymer particles. The mean sphericity(mSPHT) of these polymer particles is between approx. 0.72 and approx.0.78.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a centrifuge retention capacity (CRC) oftypically at least 15 g/g, preferably at least 20 g/g, preferentially atleast 25 g/g, more preferably at least 30 g/g, most preferably at least35 g/g. The centrifuge retention capacity (CRC) of the water-absorbingpolymer particles is typically less than 100 g/g. The centrifugeretention capacity of the water-absorbing polymer particles isdetermined by the EDANA (European Disposables and Nonwovens Association)recommended test method No. WSP 241.2-05 “Centrifuge retentioncapacity”.

The water-absorbing polymer particles are tested by means of the testmethods described below.

Methods:

The analyses should, unless stated otherwise, be conducted at an ambienttemperature of 23±2° C. and a relative air humidity of 50±10%. Thewater-absorbing polymer particles are mixed thoroughly before theanalysis.

Mean Droplet Size

The mean droplet size of the monomer solution (D50) is the droplet sizefor which exactly 50% by volume of the droplets are smaller than thisvalue, and is determined with a Malvern Insitec® S (Malvern InstrumentsLtd.; Malvern; GB). A 450 mm lens and the “RTSizer” control andevaluation software are used. The measurement ready is set to from 0.1to 2000 μm. The dropletizer plate/laser beam distance is 1.05 m, and thedroplet/lens distance is 15 cm.

Distribution Width of the Droplet Size

The distribution width of the droplet size (SPAN) is

${{SPAN} = \frac{{D\; 90} - {D\; 10}}{D\; 50}},$where D10 is the droplet size form which exactly 10% by volume of thedroplets are smaller than this value. D50 the droplet size for whichexactly 50% by volume of the droplets are smaller than this value, andD90 the droplet size for which exactly 90% by volume of the droplets aresmaller than this value. A distribution width of 0 corresponds here to amonodisperse chain of droplets. The distribution width is determinedwith a Malvern Insitec® S (Malvern Instruments Ltd.; Malvern; GB). A 450mm lens and the “RTSizer” control and evaluation software are used. Themeasurement ready is set to from 0.1 to 2000 μm. The dropletizerplate/laser beam distance is 1.05 m and the droplet/lens distance is 15cm.Mean Particle Size (Particle Size Distribution)

The mean particle size of the water-absorbing polymer particles isdetermined by the EDANA (European Disposables and Nonwovens Association)recommended test method No. WSP 220.2-05 “Particle Size Distribution”.

Moisture Content

The moisture content of the water-absorbing polymer particles isdetermined by the EDANA (European Disposables and Nonwovens Association)recommended test method No. WSP 230.2-05 “Moisture content”.

Centrifuge Retention Capacity

The centrifuge retention capacity (CRC) is determined by the EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 241.2-05 “Centrifuge Retention Capacity”.

Absorption Under a Pressure of 49.2 g/cm²

The absorption under a pressure of 49.2 g/cm² (AUL0.7psi) is determinedanalogously to the EDANA (European Disposables and NonwovensAssociation) recommended test method No. WSP 242.2-05 “Absorption underPressure”, except that a pressure of 49.2 g/cm² is established insteadof a pressure of 21.0 g/cm².

Density

The density is determined by the EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 260.2-05“Density”.

The EDANA test methods are, for example, available from the EuropeanDisposables and Nonwovens Association, Avenue Eugene Plasky 157, B-1030Brussels, Belgium.

EXAMPLES Example 1

A monomer solution comprising 31.6% by weight of sodium acrylate, 9.9%by weight of acrylic acid, 0.17% by weight of2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.085% byweight of sodium peroxodisulfate, 0.058% by weight of 3-tuplyethoxylated glyceryl triacrylate (approx. 85% by weight) and water wasdropletized. The perforated plates used as dropletizers had 6×200 μmbores, 13×230 μm bores, 9×250 μm bores, 3×270 μm bores or 2×290 μmbores.

The droplets obtained were analyzed. The results are compiled in table1.

TABLE 1 Analysis of the spray profile Diameter of Metering rate the boreper bore D50 SPAN 200 μm 0.41 kg/h 595 μm 0.57 200 μm 0.81 kg/h 512 μm0.60 200 μm 1.62 kg/h 450 μm 0.62 200 μm 3.24 kg/h 426 μm 0.99 230 μm0.54 kg/h 754 μm 0.74 230 μm 1.08 kg/h 655 μm 0.75 230 μm 2.16 kg/h 579μm 0.74 230 μm 4.16 kg/h 485 μm 1.28 250 μm 1.22 kg/h 593 μm 0.62 250 μm1.89 kg/h 564 μm 0.64 250 μm 3.24 kg/h 532 μm 0.61 250 μm 5.41 kg/h 449μm 1.20 270 μm 1.35 kg/h 608 μm 0.66 270 μm 2.16 kg/h 566 μm 0.61 270 μm3.78 kg/h 516 μm 0.60 270 μm 6.49 kg/h 481 μm 1.38 290 μm 1.62 kg/h 615μm 0.63 290 μm 2.16 kg/h 588 μm 0.59 290 μm 4.87 kg/h 578 μm 0.69 290 μm6.49 kg/h 499 μm 1.07

The results show that the droplet size rises with the diameter of thebore and falls with rising metering rate. In addition, there exists arange within which the width of the droplet size distribution (span) isindependent of the metering rate. Above a limiting metering rate, afurther increase in the metering rate leads to a bimodal particle sizedistribution and a significantly increased proportion of droplets havinga diameter of less than 100 μm.

Example 2 Noninventive

A monomer solution comprising 31.6% by weight of sodium acrylate, 9.9%by weight of acrylic acid, 0.17% by weight of2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.085% byweight of sodium peroxodisulfate, 0.058% by weight of 3-tuplyethoxylated glyceryl triacrylate (approx. 85% by weight) and water wasdropletized, in a heated dropletization tower (height 12 m, width 2 m,gas velocity 0.1 m/s in cocurrent). The monomer solution had, at 20° C.,a dynamic viscosity of 0.0065 Pa·s, a density of 1.165 g/cm³ and asurface tension of 0.039 N/m. The dropletizer plate had 20×200 μm bores.The metering rate of the mixture was from 20.5 to 28.0 kg/h. The heatingoutput of the gas preheating was regulated such that the gas outlettemperature in the dropletization tower was a constant 130° C.

The resulting water-absorbing polymer particles were analyzed. Theresults are compiled in table 3.

Example 3

The procedure was as in example 2. The dropletizer plate had 9×230 μmbores. The metering rate of the mixture was from 18.0 to 22.0 kg/h.

The resulting water-absorbing polymer particles were analyzed. Theresults are compiled in table 3.

Example 4

The procedure was as in example 2. The dropletizer plate had 6×250 μmbores. The metering rate of the mixture was from 18.0 to 20.5 kg/h.

The resulting water-absorbing polymer particles were analyzed. Theresults are compiled in table 3.

Example 5

The procedure was as in example 2. The dropletizer plate had 3×270 μmbores. The metering rate of the mixture was from 13.0 to 14.0 kg/h.

The resulting water-absorbing polymer particles were analyzed. Theresults are compiled in table 3.

TABLE 2 Settings Diameter of Number of Metering rate Example the boresbores Metering rate per bore 2*) 200 μm 20 20.5 kg/h 1.03 kg/h 200 μm 2024.0 kg/h 1.20 kg/h 200 μm 20 28.0 kg/h 1.40 kg/h 3 230 μm 9 18.0 kg/h2.00 kg/h 230 μm 9 21.0 kg/h 2.33 kg/h 230 μm 9 22.0 kg/h 2.44 kg/h 4250 μm 6 18.0 kg/h 3.00 kg/h 250 μm 6 19.0 kg/h 3.17 kg/h 250 μm 6 20.5kg/h 3.42 kg/h 5 270 μm 3 13.0 kg/h 4.33 kg/h 270 μm 3 13.5 kg/h 4.50kg/h 270 μm 3 14.0 kg/h 4.67 kg/h *)noninventive

TABLE 3 Results Mean AUL0.7 Moisture particle Example CRC psi Densitycontent size 2*) 30.2 g/g 21.2 g/g 0.50 g/cm³ 15.8% by wt. 369 μm 30.2g/g 21.0 g/g 0.50 g/cm³ 14.7% by wt. 388 μm 32.3 g/g 21.1 g/g 0.51 g/cm³13.0% by wt. 394 μm 3 33.5 g/g 22.2 g/g 0.55 g/cm³ 15.3% by wt. 387 μm32.3 g/g 22.0 g/g 0.55 g/cm³ 14.1% by wt. 379 μm 31.7 g/g 21.8 g/g 0.55g/cm³ 14.0% by wt. 384 μm 4 31.8 g/g 21.0 g/g 0.58 g/cm³ 18.4% by wt.376 μm 30.7 g/g 21.7 g/g 0.59 g/cm³ 16.1% by wt. 382 μm 32.4 g/g 22.9g/g 0.59 g/cm³ 15.9% by wt. 379 μm 5 33.0 g/g 23.1 g/g 0.61 g/cm³ 14.7%by wt. 366 μm 31.3 g/g 22.9 g/g 0.61 g/cm³ 16.3% by wt. 370 μm 28.6 g/g20.8 g/g 0.60 g/cm³ 18.2% by wt. 372 μm *)noninventive

The results show that the density rises with rising diameter of thebores. The remaining properties of the water-absorbing polymer particlesremain unchanged.

The invention claimed is:
 1. A process for producing water-absorbingpolymer particles by polymerizing droplets of a monomer solutioncomprising a) at least one ethylenically unsaturated monomer which bearsan acid group and may be at least partly neutralized, b) at least onecrosslinker, d) at least one initiator, and e) water, in a surroundinggas phase in a reaction chamber, the monomer solution being metered intoa reaction chamber via at least one bore, wherein a diameter is from 210to 290 μm per bore and a metering rate is from 0.9 to 5 kg/h per bore.2. The process according to claim 1, wherein the metering rate per bore(in kg/h) is at least−1.9269·10⁻⁷ x ³+2.3433·10⁻⁴ x ²−5.4364·10⁻² x+3.7719, wherein x is thediameter per bore (in μm).
 3. The process according to claim 1, whereinthe metering rate per bore (in kg/h) is at most5.5158·10⁻⁸ x ⁴−5.5844·10⁻⁵ x ³+2.0635·10⁻² x ²−3.2606x+1.8698·10²wherein x is the diameter per bore (in μm).
 4. The process according toclaim 1, wherein the droplets have a mean diameter of from 300 to 700μm.
 5. The process according to claim 1, wherein the monomer solution at20° C. has a dynamic viscosity of from 0.002 to 0.02 Pa·s.
 6. Theprocess according to claim 1, wherein the monomer solution at 20° C. hasa density of from 1 to 1.3 g/cm³.
 7. The process according to claim 1,wherein the monomer solution at 20° C. has a surface tension of from0.02 to 0.06 N/m.
 8. The process according to claim 7, wherein thetemperature of the monomer solution as it passes through the bore isfrom 10 to 60° C.
 9. The process according to claim 1, wherein thewater-absorbing polymer particles have a centrifuge retention capacityof at least 15 g/g.